I believe the new Field turf being put in many stadiums is causing problems for many players. Dwight Freeney is just another example. Although the Field turf is a huge improvement over the old artificial turf, it still has it's problems.
Consider the stress these D-lineman put on their feet when they explode off the line. With natural grass, there is some give as the grass tears or the sod gives way. These guys squat 700lbs...they do drills that make them more explosive off the line...and this is what happens when there is no give from the turf.
I haven't heard this theory discussed much and I'm just stating my opinion here. But what else could it be?